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No, not that QFE/QNH debate.

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No, not that QFE/QNH debate.

Old 4th Feb 2020, 17:07
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Question No, not that QFE/QNH debate.

Having read a number of forum members getting quite aerated about their favoured altimeter setting, I'm anxious to stay well out of that argument.

However, I'm trying to find a reliable formula to convert a QNH to the QFE for an aerodrome for which I know the elevation. I figured that:
QFE = QNH - (Elevation [in feet] / 3.2808399 [to convert to metres] * 0.12 [approx pressure reduction per metre at low level])
should be pretty good, but when I tried it for EGBJ (ARP elevation 101 feet) it gave me a QFE that was 4hPa (or millibars if you'd rather) lower than the QNH. Regular readers will know well that the QFE is normally given as 3hPa different. "No big deal", I hear you say - but whilst I can't fly with a level of accuracy that would make it a problem, I'd like to understand the difference. Is it that the threshold of the oft-used runways is really 6 feet lower than the ARP, or is there something important being left out of the formula? I read up a little bit about QFF, but it made my head go funny.
Your insights will be gratefully appreciated. Those wishing to debate the merits of QFE should (respectfully) look elsewhere*.

Cheers,
Z
*I'm told the expression these days is "jog on".
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Old 4th Feb 2020, 17:56
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you are using the left arrow for your calculation where you should be using the right one. If you dug a hole at your airfield and sent your QNH set altimeter down, it would not necessarily show 0 at sea level, because it is using standard atmosphere.
I.e. You mustnít use the geographic elevation but the density altitude above QNH of your airfield for conversion.

Last edited by BDAttitude; 4th Feb 2020 at 18:46.
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Old 4th Feb 2020, 18:15
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If you are flying QFE or QNH why do you want to know the elevation in metres? Runway thresholds are often a different height AMSL than ARP's as the world isn't flat, instrument approach plates show threshold elevation, but approaches are still flown QNH. One aerodrome I fly to has 20/02 with 20 threshold 21' higher than 02. Sitting on the threshold at Kemble looking in one direction is like looking up a ski slope.

Don't over think it. If your preference is QFE or QNH circuits/landings the VFR flight guides tell you the circuit height/altitude to fly, do whichever is right for you. personally I think the QFE and Regional QNH have had their day.

SND
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Old 4th Feb 2020, 19:06
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Rounding values

One other affecting part is that at least QNH is rounded down to the nearest whole hPa.

Calculation may give more than 3 hpa, but when its rounded down it comes closer to the QFE.

At lower levels it around 27 feet / hPa.
3.74 hpa, after rounding in most cases 3.

Does it have an instrument approach and does the threshold height differ from ARP ?

Edit:
Just looked at the plates:
Airport elevation 101’
Runway 27 = 87’
Runway 09 =. 74’

Problem solved.
Stop edit.

Try calling TWR and get both QFE and QNH in exact value ( including one tenth) and youll see :-)

Last edited by AAKEE; 4th Feb 2020 at 19:36.
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Old 4th Feb 2020, 19:45
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Originally Posted by BDAttitude View Post


you are using the left arrow for your calculation where you should be using the right one. If you dug a hole at your airfield and sent your QNH set altimeter down, it would not necessarily show 0 at sea level, because it is using standard atmosphere.
I.e. You mustnít use the geographic elevation but the density altitude above QNH of your airfield for conversion.
Ahh. If only they'd drawn that diagram on the article I read about QFF. What I understand from that is that the ACTUAL atmosphere on any day doesn't exhibit that nice round 0.12hPa/m gradient. Thank you!

p.s. Unless you're flying towards me , I think you may have got left and right reversed. As you point out, my calculation uses ISA (right) rather than reality.

Originally Posted by Sir Niall Dementia View Post
If you are flying QFE or QNH why do you want to know the elevation in metres?
It's that lovely thing they call SI units. That pressure reduction "constant" is per metre. Concorde engineers apparently got VERY good at swapping back and forth between metric and imperial without having to down tools and take a 2 hour lunch break. Thanks anyway Niall.

Originally Posted by AAKEE View Post
One other affecting part is that at least QNH is rounded down to the nearest whole hPa.

Calculation may give more than 3 hpa, but when its rounded down it comes closer to the QFE.

At lower levels it around 27 feet / hPa.
3.74 hpa, after rounding in most cases 3.

Does it have an instrument approach and does the threshold height differ from ARP ?

Edit:
Just looked at the plates:
Airport elevation 101í
Runway 27 = 87í
Runway 09 =. 74í

Problem solved.
Stop edit.

Try calling TWR and get both QFE and QNH in exact value ( including one tenth) and youll see :-)
All excellent and informative points - the last being the deal maker. I need to stop looking at the ARP and start looking at the threshold elevations.
This has brought out a wicked thought; it's very tempting when inbound to ask a controller for the QFE by runway just to gauge their sense of humour. I suspect if it were a TWR, they'd have me orbiting for a few dozen minutes whilst they sent someone to measure it to the nearest Pascal for me.
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Old 4th Feb 2020, 19:59
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Originally Posted by ZG862 View Post
All excellent and informative points - the last being the deal maker. I need to stop looking at the ARP and start looking at the threshold elevations.
This has brought out a wicked thought; it's very tempting when inbound to ask a controller for the QFE by runway just to gauge their sense of humour. I suspect if it were a TWR, they'd have me orbiting for a few dozen minutes whilst they sent someone to measure it to the nearest Pascal for me.

When changing....edit wrote quicker than I thought,
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Old 4th Feb 2020, 20:20
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You are thinking too much !
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Old 4th Feb 2020, 20:39
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Originally Posted by beamer View Post
You are thinking too much !
I think so.
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Old 4th Feb 2020, 20:39
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Originally Posted by ZG862 View Post
p.s. Unless you're flying towards me , I think you may have got left and right reversed. As you point out, my calculation uses ISA (right) rather than reality.
Just wanted to say, that you inserted the "altitude" (geographic elevation) from the left leg into the formula of the right leg .
The graphic is only stolen from the web. It misses which sea level is on the earths geoid (left) and which is virtual by calculation of the standard atmosphere (right). Only at an ARP at sea level they would have to be the same.
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Old 4th Feb 2020, 21:46
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This has brought out a wicked thought; it's very tempting when inbound to ask a controller for the QFE by runway just to gauge their sense of humour. I suspect if it were a TWR, they'd have me orbiting for a few dozen minutes whilst they sent someone to measure it to the nearest Pascal for me.
I've been out of active controlling for a good few years now but I fear you might be disappointed. As I recall, at airports with a precision approach, the QFE to the tenth of a hPa, and for each threshold if necessary, was displayed in front of the controller all the time. You may have been fooled because the controllers are smart enough to know which aircraft might want that level of precision and, the vast majority, that do not. Doubt that much has changed.

The only cause for a short delay might be while the controller wonders what the heck you're asking for.......or maybe says to his/her colleagues 'You're not going to believe this, but....'.
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Old 4th Feb 2020, 21:48
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@BD: I *think* I'm with you. Let's see: The elevation of EGBJ's ARP found in the AIP is its measured, geographic distance above actual, wet & salty, mean sea level. The sea level on which QNH is quoted isn't real in that its pressure of 1013.25hPa only occurs on a spherical earth at 15 degrees C in which UK is still in EU. Therefore, calculating QFE from QNH must consider where EGBJ would actually be if the world was a perfect, slightly lumpy sphere and it was warm outside - neither of which are true. Since this figure is not published in the AIP, I'm on a hiding to nothing.
However, if I want to create a model that gives more similar results to the real world, using the runway elevation instead of the ARP elevation will do the trick for EGBJ - and may or may not work for all the other airfields in UK..

@Beamer: You're right, but it makes me happy and keeps my head warm. In other news, my model for daytime temperatures using a 10 year monthly average min & max temps report and some cosines seems to work quite well. It was worth going to school after all.

@LookingForAJob: Thank you for that. The altimeters in the aircraft I fly give generalisations at best; Combined with my ham-fistedness, plus/minus a couple of hPa will make no difference at all to my circuit flying. Nice to know the tower is well-equipped though.

Last edited by ZG862; 5th Feb 2020 at 10:57.
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Old 4th Feb 2020, 23:04
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The mere fact that you're trying to calculate a conversion from feet to metres in SEVEN (!) places of decimals (micrometers) while discussing differences between QFE and QNH in the order of six feet (two metres) tells me you really don't have a realistic grasp of what it is you're trying to do.

Fella, your altimeter doesn't/can't read to an accuracy better than 30-50ft or so - why on earth are you consideriing micrometers?

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Old 4th Feb 2020, 23:40
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30 feet per Millibar has worked well for the past 50 years.
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Old 5th Feb 2020, 07:06
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Originally Posted by ZG862 View Post
@DB: I *think* I'm with you. Let's see: The elevation of EGBJ's ARP found in the AIP is its measured, geographic distance above actual, wet & salty, mean sea level. The sea level on which QNH is quoted isn't real in that its pressure of 1013.25hPa only occurs on a spherical earth at 15 degrees C in which UK is still in EU. Therefore, calculating QFE from QNH must consider where EGBJ would actually be if the world was a perfect,
Nearly there. You have either a variable atmosphere and a fixed elevation of EGBJ which is real world. Or you have a standard atmosphere and a variable elevation which is the aviation style good enough approximation. As the variance is dependent on atmospheric condition it can't be in the AIP but you would need to ask your meteorologist for the difference QNH vs QFF of the day (hour).
The thing to take away is that QNH is just as field dependent as QFE is. Colder places will have smaller QNH than wormer places in the same air mass. Can be seen in the alps regularly.

Last edited by BDAttitude; 5th Feb 2020 at 07:44.
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Old 5th Feb 2020, 13:43
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I think you're overcomplicating. The difference between QFE and QNH for a point at a particular elevation depends only on the standard atmosphere. That's because QNH is a hypothetical construct that makes an altimeter (which uses the ISA) read the elevation at that point. The number of hPa units you have to wind on to the subscale to change from 0 to the elevation does not depend on temperature. The diagram pictured above is correct. 28 ft per hPa around sea level is close enough.

At EGBJ, while the aerodrome has an elevation of 101 ft, the runways have threshold elevations between 73 and 89 ft. Hence 3 hPa difference between QNH and threshold QFE.

Last edited by bookworm; 7th Feb 2020 at 15:38.
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Old 5th Feb 2020, 16:00
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I'm trying to find a reliable formula to convert a QNH to the QFE for an aerodrome for which I know the elevation
Set the QNH initially and then wind the altimeter down by the amount of the threshold elevation. Your QFE will be displayed on the subscale and your altimeter will be reading height above threshold. Or to do it in your head:-

30 feet per Millibar has worked well for the past 50 years
I second that.

Originally Posted by The Ancient Geek View Post
Getting back to reality, I see no need for QFE. It just causes problems when someone forgets to set QNH in the cruise, all altitudes are referenced to QNH, flight levels to 1013. When landing a simple bit of mental arithmetic gives you the height above the runway for the circuit etc. Runway is 220 ft AGL, circuit is 800 ft so fly the circuit at 1020 ft QNH - simple.
Have landed on plenty of decks about 30' ASL. Certainly never landed on a runway 220' AGL.
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Old 5th Feb 2020, 17:42
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Originally Posted by ZG862 View Post


All excellent and informative points - the last being the deal maker. I need to stop looking at the ARP and start looking at the threshold elevations.
This has brought out a wicked thought; it's very tempting when inbound to ask a controller for the QFE by runway just to gauge their sense of humour. I suspect if it were a TWR, they'd have me orbiting for a few dozen minutes whilst they sent someone to measure it to the nearest Pascal for me.
If the runway thresholds are more than 7ft below the A/D elevation, they should issue 'QFE Threshold' for each runway and each of these would be displayed on the controllers METAR display; an ATIS would probably only give QFE for the runway in use.
Course if you used QNH you would find the minima for each i.a.p. is based on threshold elevation so you wouldn't end up with a plethora of pressure settings for the same airfield.
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Old 5th Feb 2020, 21:10
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It's a few years since I researched these matters so please go steady if I'm out of date!

I believe in the UK that the published aerodrome elevation is the "highest point on the landing area" whereas in many States (ICAO definition?) the published aerodrome elevation is that of the Aerodrome Reference Point which is normally around the "middle" of the airport or main runway.

As has been stated previously the pressure is normally measured to one tenth of a Hpa but if ATC are passing a pressure setting it is corrected down to the nearest whole Hpa. E.g Pressure =1010.9 Hpa would be passed as 1010 Hpa - this would account for .9 X 27.3 = 25 feet as an "error" but on the "safe" side.
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Old 5th Feb 2020, 23:14
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Thanks Bob. I read somewhere that in UK, NATS conventionally take the Aerodrome Reference Point as being in the middle of the aerodrome's longest runway.
I appreciate the further insights on rounding.
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Old 6th Feb 2020, 11:01
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Originally Posted by ZG862 View Post
Thanks Bob. I read somewhere that in UK, NATS conventionally take the Aerodrome Reference Point as being in the middle of the aerodrome's longest runway.
I appreciate the further insights on rounding.
No it's as bob says; the 'highest point on the landing area'. And it's the CAA not NATS.
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